1. Field of the Invention
This invention relates generally to a vehicle antenna and, more particularly, to a solar-ray vehicle antenna provided in the windshield of a vehicle for AM/FM radio reception, that includes a mirror wiring ground strap.
2. Discussion of the Related Art
Most modern vehicles include a vehicle radio that requires an antenna system to receive amplitude modulation (AM) and frequency modulation (FM) broadcasts from various radio stations, Present day vehicle antenna systems may include a mast antenna that extends from a vehicle fender, vehicle roof, or some applicable location on the vehicle. Although mast antennas provide acceptable AM and FM reception, it has been recognized by vehicle manufacturers that the performance of a mast antenna cannot be significantly increased, and therefore, improvements obtained in other areas of in-vehicle entertainment systems will not include reception capabilities of the mast antenna. Consequently, vehicle manufacturers have sought other types of antenna designs to keep pace with consumer demands for increased vehicle stereo and radio capabilities.
Improvements in vehicle antenna systems have included the development of backlite antenna systems, where antenna elements are formed on a rear window of the vehicle in various designs. Backlite antenna systems have provided a number of other advantages over mast antenna systems, including no wind noise, reduced drag on the vehicle, elimination of corrosion of the antenna, no performance change with time, limited risk of vandalism, and reduced cost and installation.
A new concept for antenna systems provides an antenna between the inner and outer laminated glass sheets of a vehicle windshield. U.S. Pat. No. 5,528,314, titled "Transparent Vehicle Window Antenna" issued Jun. 18, 1996 and U.S. Pat. No. 5,739,794 titled "Vehicle Window Antenna With Parasitic Slot Transmission Line," issued Apr. 14, 1998, disclose "Solar-Ray" antennas of this type, and U.S. Pat. No. 6,020,855 transparent vehicle window antenna with capacitive connection apparatus, issued Feb. 1, 2000.
FIG. 1 is a diagrammatic view of a known Solar-Ray vehicle antenna 10 of the type disclosed in the above mentioned patents laminated in a windshield 12 of a vehicle, The windshield 12 will be mounted within an opening of a vehicle body that is made of an electrically conductive metal, such as steel or aluminum, by known window mounting techniques. The windshield 12 includes a dark tinted region 18 formed along a top border of the windshield 12 that reduces glare for the vehicle operator. The translucent nature of the tinted region 18 can be used to reduce the visibility of the antenna 10.
The antenna 10 is provided in the windshield 12 as a conductive film applied to the inner surface of an outer glass of the windshield 12 to be contained between outer and inner glass layers of the windshield 12. The film of the antenna 10 is essentially transparent to visible light, highly reflective of infrared radiation, electrically conducting, and preferably has a sheet resistance of 3 ohms per square or less. An example of a suitable film material is described in U.S. Pat. No. 4,898,789 to Finlay, issued Feb. 6, 1990. The film described herein can include a first anti-reflective metal oxide layer, such as oxide of zinc and tin, an infrared reflect,on metal layer, such as silver, a primer layer containing titanium, a second metal oxide layer, a second infrared reflective metal layer, such as silver, another primer layer, a third anti-reflective metal oxide layer, and an exterior protective layer of titanium metal or titanium oxide.
The antenna 10 includes two basic elements, a horizontally elongated tuning element 20 substantially parallel to and spaced from a top edge 22 of the windshield 12, and an impedance matching element 24. The tuning element 20 is essentially rectangular. although As horizontal edges may follow the curvature of the windshield edge 22 and its comers may be rounded for a more pleasing appearance. The tuning element 20 has an effective horizontal length of an odd integer multiple of one-quarter of the wavelength to which it is tuned, and thus exhibits a zero reactive impedance at the tuned wavelength. Different tuning element configurations can be provided in different designs.
In one embodiment, the tuning element 20 is tuned to a wavelength in the center of the FM frequency band (88 MHz-108 MHz), such as 3 meters, and thus has an effective horizontal length of about 0.75 meters. The physical length of the element 20 at resonance is actually somewhat shorter than one-quarter of the center frequency of the FM band to provide coupling to the vehicle body. The length by which the element 20 is shorter will vary with the specific vehicle application. In one particular vehicle, the tuning element 20 has been found to work well with a horizontal length of 60 cm and a vertical width of 50 mm. The element 20 is ideally spaced below the windshield edge 22 by a distance which provides maximum FM gain. However, this distance may be varied to provide other advantages for a particular vehicle design. The antenna 10 provides AM reception through capacitive coupling with the vehicle body.
The impedance element 24 includes a main body portion 28 which covers substantially all or most of the windshield 12 below the tinted region 18 to provide FM impedance matching. In the '794 patent, the impedance element can be a ribbon in various configurations to form a parasitic slot transmission line for FM impedance matching purposes. The main portion 28 has a peripheral edge 32 with a horizontal upper portion 34 spaced at least 25 mm below the lower edge of the element 20, so as to minimize transmission coupling effects therebetween. The upper portion 34 is connected to the element 20 by a narrow vertical portion 36 to provide an electrical current flow. The upper portion 34 of the peripheral edge 32 is preferably within the tinted region 18 of the windshield 12 along its entire length from one side to the other side of the windshield 12, so that the tinted region 18 overlaps the main portion 28 of the element 24. The remaining portion of the peripheral edge 32 is spaced a certain distance from the edge of the vehicle body so as to provide, in combination therewith, a planar slot transmission line that is parasitically coupled to the element 20. In one embodiment. the distance between the edge of the vehicle body and the main portion 28 is preferably within the 10-25 mm range. The length of the slot is substantially an integer multiple of one-half of the wavelength to which the tuning element 20 is tuned, so that each end of the slot transmission line, at the junctions of the upper portion 34 and the remaining portion of the peripheral edge 32, appears as an electrical open circuit.
The impedance element 24 is used to adjust the real component. of the antenna's impedance to match the characteristic impedance, typically 125 ohms, of the coaxial cable used to feed the antenna 10. This is accomplished by the predetermined width between the remaining portion of the peripheral edge 32 and the adjacent portion of the edge of the windshield. For appearance purposes, and to maximize the infrared reflecting efficiency of the windshield 12, an opaque painted band 40 may be provided around the sides and bottom of the windshield 12 to substantially or completely cover the area outward from the remainder portion of the peripheral edge 32 to the outer edge of the windshield 12. This band can be broken into dots of decreasing size towards the inner boundary for a fade-out effect, as known in the industry. If such a band is provided in combination with the tinted region 18, substantially the entire viewing area of the windshield 12 can be uniformly provided with the infrared reflecting film of the antenna 10.
The impedance element 24 also provides an added benefit at AM wavelengths. At these longer wavelengths, the antenna 10 is not a resonant antenna, but is substantially a capacitive antenna. The large area of the element 24 provides a substantial boost in gain for the antenna 10, as compared with similar planar and other antennas in the prior art. In fact, the boost in AM gain is so great that some of it can be sacrificed, if desired, in fine tuning the antenna performance for further improvements in FM gain, directional response, or other characteristics while still yielding good AM performance.
In order to connect the antenna 10 to a radio or other communications system, a connection arrangement is necessary for an external coaxial cable. An inner conductor 42 of a coaxial cable 44 is electrically connected to a planar capacitor grid feed 46 formed on an inside surface of the inner layer of the windshield 12. The capacitor grid feed 46 makes a capacitive feed connection to the tuning element 20 through the inner glass layer. An outer conductor of the coaxial cable 44 is connected to the vehicle body at a convenient point close to where the inner conductor 42 is coupled to the feed point. Any suitable feed connection can be provided between the capacitor grid feed 46 and the center conductor 42 of the coaxial cable 44 within the skill of the art.
The above described solar-ray antenna is currently being used in certain vehicles, and has been proposed to be used in certain future vehicles. However, for one of the proposed vehicles, two new electrical systems will be added to the vehicle that will have an adverse effect on the performance of the known solar-ray antenna. These two systems include a factory installed On-Star system and an interior rear-view mirror lighting system. These systems require a multi-wire bundle that extends down from the front center edge of the roof to the rear-view mirror that is attached to the windshield. In this configuration, the multi-wire bundle will travel directly across the upper part of the known solar-ray antenna and its feed system, affecting antenna reception. Particularly, the FM signals received by the antenna will RF couple to the wire grid bundle and adversely affect the antenna performance. The effects of this coupling may be different from vehicle to vehicle, depending on movement of the wire bundle relative to the antenna elements and the load on the wires in the bundle.
What is needed is a modified design of the known solar-ray antenna so that the multi-wire bundle will not interfere with the antenna reception. It is therefore an object of the present invention to provide such an improved solar-ray antenna.